The effects of proteins on biological inactivation of DNA by ultraviolet radiation and chemical carcinogens are being studied. The loss of transfecting ability of viral DNA provides a quantitative assay for these effects. Amino acids and oligopeptides of known amino acid composition are being used to determine which properties of a protein are most important in determining its effect on DNA sensitivity. Correlations between the extents of interaction of particular proteins or oligopeptides with DNA and their effects on sensitivity are being sought using solvent perturbation techniques. Cellular mechanisms for repair of DNA-protein crosslinks are being explored. With this system, the effects of carcinogens and combinations of carcinogens on transfecting DNA can be separated from effects on the host cells. DNA treated in the presence of proteins is being assayed for the extent of damage produced by irradiation or chemical carcinogens. The extent of damage is being correlated with loss of biological activity.